An unpatched vulnerability in the Magento e-commerce platform could allow hackers to upload and execute malicious code on web servers that host online shops.

The flaw was discovered by researchers from security consultancy DefenseCode and is located in a feature that retrieves preview images for videos hosted on Vimeo. Such videos can be added to product listings in Magento.

The DefenseCode researchers determined that if the image URL points to a different file, for example a PHP script, Magento will download the file in order to validate it. If the file is not an image, the platform will return a "Disallowed file type" error, but won't actually remove it from the server.

An attacker with access to exploit this flaw could achieve remote code execution by first tricking Magento to download an .htaccess configuration file that enables PHP execution inside the download directory and then downloading the malicious PHP file itself.

Once on the server, the PHP script can act as a backdoor and can be accessed from an external location by pointing the browser to it. For example, attackers could use it to browse the server directories and read the database password from Magento's configuration file. This can expose customer information stored in the database, which in the case of online shops, can be very sensitive.

The only limitation is that this vulnerability cannot be exploited directly because the video-linking functionality requires authentication. This means attackers need to have access to an account on the targeted website, but this can be a lower-privileged user and not necessarily an administrator.

The authentication obstacle can also be easily overcome if the website doesn't have the "Add Secret Key to URLs" option turned on. This option is intended to prevent cross-site request forgery (CSRF) attacks and is enabled by default.

CSRF is an attack technique that involves forcing a user’s browser to perform an unauthorized request on a website when visiting a different one.

"The attack can be constructed as simple as <img src=… in an email or a public message board, which will automatically trigger the arbitrary file upload if a user is currently logged into Magento," the DefenseCode researchers said in an advisory. "An attacker can also entice the user to open a CSRF link using social engineering."

This means that by simply clicking on a link in an email or by visiting a specifically crafted web page, users who have active Magento sessions in their browser might have their accounts abused to compromise websites.

The DefenseCode researchers claim that they've reported these issues to the Magento developers back in November, but received no information regarding patching plans since then.

Several versions of the Magento Community Edition (CE) have been released since November, the most recent one being 2.1.6 on Tuesday. According to DefenseCode, all Magento CE versions continue to be vulnerable, which is what prompted them to go public about the flaw.

“We have been actively investigating the root cause of the reported issue and are not aware of any attacks in the wild,” Magento, the company that oversees development of the e-commerce platform, said in an emailed statement. “We will be addressing the issue in our next patch release and continue to consistently work to improve our assurance processes.”

"All users are strongly advised to enforce the use of 'Add Secret Key to URLs' which mitigates the CSRF attack vector," the DefenseCode researchers said. "To prevent remote code execution through arbitrary file upload the server should be configured to disallow .htaccess files in affected directories."

Magento is used by over 250,000 online retailers, making it an attractive target for hackers. Last year, researchers found thousands of Magento-based online shops that had been compromised and infected with malicious code that skimmed payment card details.